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CiS 260: App Dev I. Chapter 5: Methods and Classes. Introduction to Methods. A method (or ________) is a segment of code that performs a specific task. Advantages of modular program design: allows you to focus on a small part of the program many programmers can work on different methods

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cis 260 app dev i

CiS 260: App Dev I

Chapter 5:

Methods and Classes

introduction to methods
Introduction to Methods
  • A method (or ________) is a segment of code that performs a specific task.
  • Advantages of modular program design:
    • allows you to focus on a small part of the program
    • many programmers can work on different methods
    • encourages software reuse
    • improves the simplicity and readability of programs
  • Java provides ___________ methods in collections of classes called class __________.
  • Examples of predefined methods are println(), showInputDialog(), setText(), etc.
  • The programmer can create his own ___________ methods in classes he creates.
predefined methods
Predefined Methods
  • An algebraic function looks like f( x ) = 2x + 5
    • f is the name of the function
    • 2x + 5 is the body of the function
    • x is the parameter of the function, any real number
    • if x = 2, we write f( 2 ) and 2 is the argument of f
    • f(2) = 2 * 2 + 5 = ___.
  • Java has a pow(x, y) method in the Math class
    • pow is the name of the method
    • x and y are the ____________ of the method, both of type double
    • For the arguments x=2 and y=3, Math.pow(2, 3) means 2 raised to the power 3, or ___
  • You often need to use the import statement.
user defined methods
User-Defined Methods
  • Predefined methods are prewritten to perform common, basic tasks.
  • User-defined methods are ________ designed to perform very specialized tasks.
  • Two kinds of user-defined methods
    • value-returning (returns a result to the calling method)
    • void (doesn’t return a result, just executes)
value returning method
Value-Returning Method
  • Every method has these five parts
    • name (such as pow or myCustomMethod)
    • parameter list (in parentheses)
    • ______ type of each parameter, like double
    • the data type of the returned value, like double
    • the program code executed by the method
  • The first four above constitute the method __________ and the last is called the ______.
  • The five components above constitute a method definition.
  • To use a method in a program, you must code a method _______.
method example
Method Example
  • Suppose you wrote the following method

public static double raiseToAPower (double base, double exponent)

{

double result = 1;

for( int i = 1; i <= exponent; i++ )

result *= base;

return result;

}

  • public and static are modifiers, double is the return type, and raiseToAPower is the ______.
  • (double base, double exponent) is the parameter list and the code inside the {}’s is the ______.
class with a user defined method
Class With a User-Defined Method

public class Power

{

public static void main( String[] args )

{

double answer;

answer = raiseToAPower( 2,3 ); // method ______

System.out.println(answer); // displays 8.0

} // end main

public static double raiseToAPower (double base, double exponent)

{

double result = 1;

for( int i = 1; i <= exponent; i++ )

result *= base;

return result; // return this value to main

} // end raiseToAPower

} // end class

the return statement
The Return Statement
  • The syntax of a return statement in a method is returnexpr; where expr is a variable, constant value, or ____________.
  • The data type of the value of expr must match the _______ type of the method.
  • When a return statement executes, the method terminates and control goes back to the ________.

public static double larger( double x, double y )

{

double max;

if( x >= y )

max = x;

else

max = y;

return max;

}

flow of execution
Flow of Execution
  • The order of methods in a class doesn’t matter.
  • When a program executes, the first statement in the method ________ executes first.
  • When the last statement of a method executes, control is passed back to the point immediately following the method call.
  • For a value-returning method, the value that the method returns replaces the method call and execution follows immediately after the call.
  • A _________statement can be used in a void method to exit a method early.
  • To call a void method, use methodName();
data types and parameters
Data Types and Parameters
  • For primitive data types, variables are passed from one method to another by ______ .
  • This means methods have their own copies of primitive data separate from each other.
  • For reference variables (objects or abstract data types) variables are passed by __________.
  • Therefore, a change in the value for a reference variable in one method, automatically changes it in any other method.
  • Remember that a variable of type String is a reference variable (points to a String _______).
scope of an identifier
Scope of an Identifier
  • The scope of an identifier is that part of the program in which it is accessible (________).
  • A ______ identifier is declared within a method or block and is visible only within that method or block.
  • When a counter variable is declared in a for loop, its scope is limited to the for loop.
method overloading
Method Overloading
  • In Java, a class can contain several methods with the same name.
  • However, methods with the same name must have different _________________.
  • This is called method ____________.
  • This makes the naming of methods easier.
the class string
The class String
  • In Java, a string is not a variable, but an _____.
  • The following two statements are equivalent:
    • String name = “Lisa Johnson”;
    • name = new String( “Lisa Johnson” );
  • nameis actually a reference variable that contains the address of the String ________.
  • A string contains ___ or more characters enclosed in double quotes.
  • The ________ of the first character in a string is 0, of the second character is 1, and so on.
methods in the class string
Methods in the class String
  • The method substring() is a member of class String.
  • Using String name = “Lisa Johnson”; :
    • name.substring(0,4) yields _______
    • name.substring(5,12) yields “Johnson”
    • name.indexOf(‘J’) yields ___
    • name.charAt(4) yields ‘ ’
    • name.equals(“Lisa Johnson”) yields true
    • name.equals(“Luke Johnson”) yields ______
    • name.length() yields ____
    • name.replace(\'i\',\'e\') yields “Lesa Johnson”
  • The String object name has access to the String methods using the ____ operator.
formatting numbers
Formatting Numbers
  • You can format numbers in your program using complicated program logic (see p. 226)
  • Or you can use Java pre-defined methods:
    • NumberFormat decimal = NumberFormat.getNumberInstance();
    • decimal.setMaximumFractionDigits(2);
    • String aDecimalStr = decimal.format(19.857); //19.86
    • NumberFormat percent = NumberFormat.getPercentInstance();
    • String aPercentStr = percent.format(.857); // 86%
    • NumberFormat currency = NumberFormat.getCurrencyInstance();
    • String aCurrencStr = currency.format(19.95); //$19.95
the calendar class
The Calendar Class
  • Calendar today = Calendar.getInstance();
  • int month = today.get( Calendar.MONTH ) + 1; // returns numeric month (1 for Jan, 2 for Feb, …)
  • Date todaysDate = today.getTime();
  • System.out.print( todaysDate ); // prints date & time
  • Calendar dueDate = Calendar.getInstance();
  • dueDate.set( 2007, 11, 31 ); // due date is Dec. 31
the gregoriancalendar class
The GregorianCalendar Class
  • GregorianCalendar birthDate = new GregorianCalendar( 1953, 8, 10 ); // Sept. 10, 1953
  • int dayOfWeek = birthDate.get( GregorianCalendar.DAY_OF_WEEK ); // returns, for example, Tuesday
  • Long birthTime = birthDate.getTimeInMillis(); // time in milliseconds
  • DateFormat longDate = DateFormat.getDateInstance( DateFormat.LONG );
  • String birthDayStr = longDate.format( birthDate ); // “September 10, 1953”
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